The present invention relates to mine roof supports, and more particularly to the type of mine roof support with a vertically adjustable roof shield.
Mine roof supports of this type are already known in general. They have a base on which a roof shield is pivoted, the upper end of the shield being provided with a cap that is pressed by a hydraulic pit prop against the roof of the mine gallery or the like. A free edge of the cap is, in operation of the support, so positioned that it is close to the mine face so that the workers and equipment beneath the cap are protected against break-in of the roof of the gallery. An advantage of this type of mine roof support is the fact that the shield together with the mounting arrangement, the base and the pit prop forms in effect a four-bar linkage, a construction which assures that over a certain height of the mine gallery the roof cap can be adjusted vertically in such a manner that its front edge adjacent the mine face itself moves in an almost completely vertical path and thus can always remain adjacent the mine face and provide proper security for the personnel working below it. However, this vertical adjustment range is limited, i.e. if the vertical height of a coal seam requires the mining gallery to be higher than a predetermined limit, further upward adjustment of the roof cap is either not possible or, if it can be done, causes the front edge of the roof cap to move rearwardly away from the mine face leaving a portion of the mine roof unsupported. The reason for this has to do with the particularities of supporting underground passages, in particular the required or desired inclination of the roof shield and the distance at which the lower end of the shield proximal to the base must be spaced from that end of the base which is distal from the mine face. The roof shield must not be inclined at too flat an angle to the vertical because otherwise forces acting upon it from the roof and the overhang will negatively influence the supporting capability of the mine roof support. Also, when the roof shield is retracted to a relatively low position, the lower end of the roof shield must not project too far beyond that end of the base which faces away from the mine face, because otherwise the thus projecting lower end would conflict with rubble which drops into the goaf and readjustments of the roof shield would become difficult or impossible because of the presence of such rubble. In such an event, also, the linkages connecting the roof shield to the base, and the journals for the linkages, would be subjected to excessively high stresses.
Theoretically it would be possible to increase the height over which the front edge of the roof cap can be adjusted in a substantially vertical path, by reconstructing the mine roof support with respect to the length of the linkages and of the roof shield, as well as with respect to the position at which the linkage are connected to the roof shield and to the base. However, if this is done then necessarily the roof shield and the linkages must be increased in size and this, in turn, means that when the roof shield is lowered to a position in which it is relatively close to the face, it will project rearwardly so far into the goaf that its subsequent readjustment will be most disadvantageously influenced, for the reasons explained above. Also, if the mine gallery is of relatively small height a so reconstructed mine roof support would necessarily have its roof shield inclined at so flat an angle to the vertical that the other disadvantages outlined above would inherently result.
To avoid these problems mine roof supports of the type in question, i.e. having a four-bar linkage construction, heretofore have been designed so that they can operate within a predetermined limited mine gallery height. Within this predetermined limited height the disadvantages outlined above can be eliminated. These supports could not, however, be used if the mine gallery height was greater than the predetermined limited height unless one was prepared to accept the aforementioned disadvantages.